In electrophotography, an organophotoreceptor in the form of a plate, belt, disk, or drum having an electrically insulating photoconductive element on an electrically conductive substrate is imaged by first uniformly electrostatically charging the surface of the photoconductive layer, and then exposing the charged surface to a pattern of light. The light exposure selectively dissipates the charge in the illuminated areas where light strikes the surface, thereby forming a pattern of charged and uncharged areas (referred to as latent image). A fine liquid or solid toner is then provided in the vicinity of the latent image, and toner droplets or particles deposit in either the charged or uncharged areas to create a toned image on the surface of the photoconductive layer. The resulting visible toner image can be transferred to a suitable permanent or intermediate receiving surface such as paper, or the photoconductive layer can operate as a permanent receptor for the image. The imaging process can be repeated many times to overlay images of distinct color components or effect shadow images, such as overlaying images of distinct colors to form a full color final image.
Both single layer and multilayer photoconductive elements have been used commercially. In the single layer embodiment, a charge transport material and charge generating material are combined with a polymeric binder and then deposited on an electrically conductive substrate. In the multilayer embodiment, the charge transport material and charge generating material are present in the element in separate layers, each of which materials can optionally be combined with a polymeric binder and deposited on the electrically conductive substrate. Two arrangements are possible. In one arrangement (the “dual layer” two layer arrangement), the charge generating layer is deposited on the electrically conductive substrate and the charge transport layer is deposited on top of the charge generating layer. In an alternate arrangement (the “inverted dual layer” two layer arrangement), the order of the charge transport layer and charge generating layer is reversed.
In both the single and multilayer photoconductive elements, the purpose of the charge generating material is to generate charge carriers (i.e., holes or electrons) upon exposure to light. The purpose of the charge transport material is to accept these charge carriers and transport them through the charge transport layer in order to discharge a surface charge on the photoconductive element.
To produce high quality images, particularly after multiple cycles, it is desirable for the charge transport material to form a homogeneous solution with the polymeric binder and remain in solution. In addition, it is desirable to maximize the amount of charge which the charge transport material can accept (indicated by a parameter known as the acceptance voltage or “Vacc”), and to minimize retention of that charge upon discharge (indicated by a parameter known as the residual voltage or “Vres”).
There are many charge transport materials available for electrophotography. The most common charge transport materials are pyrazoline derivatives, fluorene derivatives, oxadiazole derivatives, stilbene derivatives, hydrazone derivatives, carbazole hydrazone derivatives, triphenylamine derivatives, julolidine hydrazone derivatives, polyvinyl carbazole, polyvinyl pyrene, or polyacenaphthylene. However, each of the above charge transport materials suffers some disadvantages. There is always a need for novel charge transport materials to meet the various requirements of electrophotography applications.